Sheet metal piece parts such as used in automobiles, appliances, aircraft, farm implements, construction equipment, etc., are typically formed by a series of stamping operations in a multi-stage mechanical transfer press which manipulates the workpiece to a desired shape and size. Each stamping operation makes use of a pair of dies which engage the sheet metal piece part and form it as desired.
A process known as stretch forming has been gaining increasing acceptance in shaping and sizing of sheet metal piece parts. In this approach, the piece part is engaged around its peripheral edge by upper and lower holding rings which maintain the piece part in fixed position while stretching it on a fixed lower die. A movable upper die then stamps the piece part in the desired size and shape. A primary advantage of this stretch forming approach are a reduction in piece part size and an associated cost savings. Another advantage of this approach is in a more uniform stretching of the piece part over its entire surface rather than greater stretching in the corners of the piece part in comparison with its inner portion as in prior art approaches. The more uniform stretching of the piece part reduces so-called "loose metal", or "flutter". As a result, the stretch forming technique provides higher quality parts with more consistent and predictable properties.
Prior art stretch forming approaches to metal piece part stamping are not without limitations. One problem encountered in the prior art arises from the high impact force between the moving upper holding ring and the lower holding ring in pinching the outer peripheral edge of the piece part. At this stage in the stamping process, the upper holding ring is in mid-stroke and traveling at high speed giving rise to excessively high impact forces which frequently are well beyond the rated tonnage capacity of the press resulting in excessive wear and damage to the press necessitating costly and time consuming repair.
This invention addresses the aforementioned limitations of the prior art by providing for high impact force absorption in a stamping press for isolating press components from these impact forces which may exceed the rated tonnage capacity of the press and cause damage to or destruction of press components.